PROJECT SUMMARY. Nearly half of HIV+ patients will experience symptoms of HIV-associated neurocognitive deficits, which are further exacerbated by opiate drug addiction. Opiate drugs impact neurons through direct (i.e., excitotoxicity) and indirect (i.e., glial inflammation) mechanisms. However, while certain brain regions such as striatum exhibit dendritic swelling and degeneration, the hippocampus exhibits a more subtle pathology: transient alterations in intracellular calcium ([Ca2+]i), and sodium ([Na+]i), reduced LTP, and declines in specific CA1 interneuron subtypes, in the absence of gross neurodegeneration. Our preliminary work revealed that GABAA agonism prevented Tat-induced [Ca2+]i transients in vitro, while GABAA antagonism prolonged morphine/Tat-mediated [Ca2+]i elevation. We hypothesize that morphine exacerbates HIV-induced neuronal dysfunction through its influence on the inhibitory microcircuitry in the hippocampus, leading to a net disruption in the excitatory/inhibitory balance of CA1 pyramidal cells and eventual neuronal injury. To study this, we propose to assess the impact of GABAA signaling on CA1 pyramidal cells. In Aim 1, we will use Camk2a- Cre/TdTomatoflox/flox CA1 pyramidal cell reporter mice to assess the impact of GABA signaling on HIV- Tat/morphine-induced changes in CA1 pyramidal cell ion homeostasis (Ca2+ and Cl-), physiology, structure and survival in vitro following exposure to morphine, HIV-Tat, and GABAA/B agonists/antagonists in vitro. Fluorescent ion indicator results will be confirmed with whole-cell, patch-clamp electrophysiology of these dissociated cells. These observations will be confirmed in human hippocampal neurons, exposed to HIV-Tat or intact, infectious HIV (HIVBaL). In Aim 2, we will utilize Cre-inducible DREADD (Designer receptors exclusively activated by designer drugs) transgenic mice crossed with HIV-Tat transgenic mice to study the impact of acute and chronic modulation of overall interneuron excitation or inhibition on CA1 pyramidal neuron physiological and morphological outcomes in the Tat tg model. We propose that reduced GABAergic input could further HIV-Tat-induced excitotoxic damage, leading to prolonged physiological and structural alterations within hippocampal circuitry. By adjusting GABAergic gain within this system, we could ameliorate Tat and opiate-induced hippocampal neurodegeneration and dysfunction. Through the proposed combination of electrophysiology and chemogenetic manipulation, we will further our understanding of opiate/Tat-mediated neuropathology within the hippocampus.